An Equation for the Adsorption of a Solute on the Air/liquid or Liquid/liquid Interface and its Application to Relevant Problems

On the basis of Langmuir's. theory, an equation for the adsorption of an electrically neutral solute on the air/liquid or liquid/liquid interface has been derived. Using this isotherm, an expression for the distri­bution of a solute between two immiscible liquids (molecular weight of the solute remaining constant) has been found in agreement with the Nernst distribution law. Combining this isotherm with that of Gibbs, an equation connecting interfacial tension, \(\gamma\), with the concentration of the solute in water has been derived. This equation has the same form as the Szyazkowaki empirical equation for the lowering of surface tension of aqueous solutions by the lower fatty acids. The application of the isotherm has been emended to the adsorption of surface-active electrolytes at the interface of oil/water. It has been shown that dπ / dlog C=2.502 kT/cm 59 mV at 25°. The application of the isotherm has also been extended to account for the variation of surface pressure ac with the surface area A per molecule of surface-active electrolytes. Correction for the effect of electrical charge has been introduced in two steps by showing (a) that the equation giving π should be multiplied by a factor '2' and (b) that π should be increased by Δπ and finding an cameo-lion for the latter (i.e. Δπ) under certain conditions. The corrected equation has been found to agree fairly with the data obtained by Cockbain on the adsorption of sodium dodecyl sulphate (NaLS) at the ohs:me/water interface